Silicon wafers used for the manufacture of high efficiency photovoltaic cells are generally derived from ingots made of monocrystalline silicon obtained by the Czochralski (Cz) pulling method. These wafers are preferentially n-typed doped, by introducing electron donor phosphorous atoms into the silicon. An n-type doping with phosphorous (P) is preferred to a p-type doping with boron (B), notably because of the absence of boron-oxygen complexes that reduce the lifetime of the charge carriers in the silicon.
Phosphorous is incorporated in the silicon before the pulling of the ingot, when the silicon is in the molten state, by adding to the silicon melt a phosphorous powder or silicon wafers highly doped with phosphorous. This doping technique has drawbacks, which include the contamination of the silicon. In fact, the phosphorous powder or the silicon wafers added to the melt also contain metals and carbon. These impurities are incorporated in the silicon at the same time as the phosphorous, which leads to a contamination of the silicon, firstly at the level of the melt, then at the level of the ingot. Moreover, given that the segregation coefficient of phosphorous is low (around 0.35), a significant variation in the phosphorous concentration, and thus in the electrical resistivity, appears over the height of the ingot. Yet the performances of photovoltaic cells are dependent on the electrical resistivity. Photovoltaic cells obtained from the ingot will then not have the same performances, and notably the same photovoltaic conversion efficiency. Thus, a portion of the ingot risks being unusable, which represents a financial loss for the wafer supplier.
To avoid these drawbacks, another doping technique involving thermal donors has been developed. It is described in the article [“High Quality Thermal Donor Doped Czochralski Silicon Ingot for Industrial Heterojunction Solar Cells”, F. Jay et al., EU PVSEC Proceedings 2015, pp. 316-321]. The thermal donors are agglomerates created from the interstitial oxygen contained in the silicon (i.e. the oxygen atoms occupy interstitial positions in the crystal lattice), when it is subjected to a temperature comprised between 350° C. and 550° C. Each thermal donor generates two free electrons, which produces a variation in the electrical resistivity of the silicon.
This doping technique enables wafers of practically identical resistivity and only containing very few impurities to be obtained. Firstly, an ingot made of monocrystalline silicon is crystallised from a silicon melt, by means of the Czochralski method. The silicon used to prepare the melt is intrinsic and no dopant has been voluntarily introduced into the melt. In these conditions, the resistivity of the ingot only depends on the thermal donors concentration. The interstitial oxygen concentration and the initial concentration of thermal donors, formed during the crystallisation, are then measured over the height of the ingot. A concentration of additional thermal donors, to be created to reach a target resistivity, may then be calculated, for each height of the ingot. These additional thermal donors are formed during an annealing at 450° C. For each height of the ingot, the annealing time required to obtain additional thermal donors is calculated, knowing the interstitial oxygen concentration. The ingot is then cut into sections. Each section corresponds to an annealing time, because the calculated annealing times are substantially identical in a same section. Finally, each section of ingot is subjected to annealing at 450° C. for the corresponding time, before being cut into wafers.
The most critical step of this wafer manufacturing method is the annealing at 450° C. of the different ingot sections. It is necessary in fact to avoid sudden to variations in temperature when a section is introduced into the annealing furnace, then taken out of the furnace, because the section can break. Consequently, it is advisable to increase progressively the temperature after having introduced the section into the furnace, and to reduce it just as progressively before extracting the section from the furnace. The problem is that it is then difficult to know the exact time of formation of thermal donors. In fact, thermal donors are also formed during the heating phase (from 350° C. to 450° C.) and the cooling phase (from 450° C. to 350° C.) of the furnace. The quantity of additional thermal donors formed during the annealing then differs from that calculated from the target resistivity and the target resistivity is not finally reached.